1. Field of the Invention
The present invention involves the loading of calcium carbonate into fibers contained in a pulp suspension.
2. Description of the Related Art
Pulp suspensions are used especially in paper and cardboard manufacture. The sparing use of raw material resources, due especially to economical and ecological concerns, is reflected in the paper production industry by the use of paper webs with lower basis weights, as well as by the partial replacement of pulp with filling materials. If lower cost raw materials are used, the paper quality should at least be maintained. Among other things, the end product""s strength, visual characteristics, and processability play key roles in this challenge.
The present invention relates to the further optimization of the paper production process, especially with regard to achieving the greatest possible profitability and the highest possible pulp suspension quality.
The invention, in one form thereof, comprises a process of adding to the pulp suspension a medium containing calcium oxide and/or calcium hydroxide. The pulp suspension subjected to this treatment is additionally charged in at least one reactor with pure carbon dioxide or a medium containing carbon dioxide. During the course of the chemical reaction, at least a significant portion of the above-mentioned starting materials (calcium oxide and/or calcium hydroxide and carbon dioxide) is converted into the reaction products of calcium carbonate and water. This conversion is achieved by accordingly controlling the pulp suspension""s pH.
The pH can be measured at one or several locations during execution of the process. The pulp suspension is further characterized by a material density (i.e., consistency) greater than 5%, and preferably between 15% and 40%. The density is also controllable within this range.
The addition of a medium containing calcium oxide and/or calcium hydroxide results in an exothermic reaction. Liquid calcium hydroxide (lime milk) is preferable for this application. That the reaction is exothermic in nature means that the water settled in or on the pulp suspension""s fibrous material is not necessarily required to start and continue the chemical reaction. Significantly greater profitability and higher quality pulp suspension are achieved as a result of this reaction.
During loading of the fibers, calcium carbonate is deposited onto the wetted fiber surfaces through the addition of calcium oxide and/or calcium hydroxide to the moist fibrous material, whereby at least a portion of this calcium compound can associate with the water in the fibrous material. After this treatment, the fibrous material is charged with the pure carbon dioxide or with the medium containing carbon dioxide. Moreover, the CaCO3 that is formed can create a suspension around the fibers. Accordingly, the fibers are loaded with the filling material calcium carbonate, whereby deposition onto the wetted fibrous surfaces is performed according to a so-called xe2x80x9cFiber Loading(trademark) Processxe2x80x9d, as described in document U.S. Pat. No. 5,223,090. During this xe2x80x9cFiber Loading(trademark) Processxe2x80x9d, Co2 reacts with calcium hydroxide to form water and calcium carbonate.
The term xe2x80x9cwetted fiber surfacexe2x80x9d can include the wetted surfaces of all individual fibers. This applies especially to cases where the fibers are loaded with calcium carbonate on their external as well as on their inner surfaces (lumen).
A preferred version of the invention process compares the respective pH value with a corresponding preset value and minimizes or eliminates the control deviation through manipulation of at least one of the following process variables:
Length of time pulp suspension remains in the reactor;
Pulp suspension feed rate;
Carbon dioxide pressure;
Temperature of the pulp suspension and/or the calcium compound;
Pressure inside the reactor;
Temperature of the CO2;
Pressure of the CO2;
Concentration of CO2 in the medium;
Concentration of the CaO, the Ca(OH)2, and the fibers; and
Specific fiber surface area.
It is advantageous to maintain control of the pulp suspension pH within a range of about 5.5 to 10.5.
It is also advantageous when the pulp suspension""s ash content is controllable within a range of about 1% to about 70%.
It is preferable to feed the carbon dioxide in a gaseous state. Further, the temperature of the fed carbon dioxide can be adjusted or controlled within a range of about 10xc2x0 C. to about 250xc2x0 C.
In certain cases, it is also advantageous to use other visual characteristics such as brightness, light scattering properties, opacity, color location, and the light dispersion coefficient as indicators of the status of the chemical reaction.
The pH should be measured during at least one of the following steps: at least before and after the reaction; during the reaction; and possibly multiple measurements throughout (optional).
It is preferable to measure the pH at the end of the chemical reaction or following the enlargement of the specific surface area, such enlargement being achieved by using as least one fluffer.
The pressure can be controlled within the range of about 0.1 to about 20 bar.
Furthermore, the pulp suspension is subjected to shearing force, preferably in at least one fluffer, in order to enlarge its specific surface area, among other things.
Furthermore, loading the fibers with calcium carbonate can be accomplished as described in document U.S. Pat. No. 5,223,090, the contents of which are hereby incorporated by reference.